1. Field of the Invention
The present invention relates to the technologies of ultra-high speed photography, and more particularly to a camera bellows of a rotating-mirror framing camera without principle error.
2. Description of the Related Technology
A high speed photographic apparatus is an extension of a time resolution capability of a human eye and can record spatiotemporal information within transient processes, thus providing a powerful tool for analyzing and investigating the transient processes. To investigate ultra-fast processes in such fields as detonation, fusion, discharge, high-speed combustion, matter effects of high-power laser acting, non-stable vortex, meso-mechanics or micro-mechanics effects, a high speed photographic apparatus having time resolutions of microsecond and sub-microsecond is generally used. Among various high speed photographic apparatuses that satisfy these above requirements, a rotating-mirror framing camera, which has such characteristics as bigger frame size, higher frame count, higher spatial resolution, and broader framing frequency bandwidth, can handle most of the research tasks that involve ultra-fast processes at an ultra-high framing frequency of 104˜107 fps (frames per second). Therefore, the point of focus in the research field of ultra-high speed photography has the tendency to return back to the rotating-mirror framing camera after transitioning from a rotating mirror model to an image converter tube model and a solid image device model. This framing camera has always been a focal point of research on ultra-high speed photography (SPIE, 2003, Vol. 4948: 330˜335; SPIE, 2007, Vol. 6279: 62791U-1˜62791U-9).
The conventional camera bellows of the rotating-mirror framing camera is normally designed based upon the Miller theory. As shown in FIG. 1, a camera bellows of a rotating-mirror framing camera that is built upon the Miller theory generally includes a box, and inside the box, a rotating mirror 4, a relay lens array 5 as well as an image recording surface 6. A primary image I1 that is captured by an object lens 1, which is outside the box, passes through an ocular lens 2 and a field lens 3 to produce a secondary image I2. By adjusting the position of the field lens 3, the secondary image I2 is formed at a certain location that is adjacent to the rotating mirror 4. A mirror image I2′ is formed upon mirroring the secondary image I2 by the rotating mirror 4. Through the relay lens array 5, a final image I3′ of the mirror image I2′ of the secondary image I2 is formed on the image recording surface 6. The mirror image I2′ of the secondary image I2 and the final image I3′ of the image recording surface form an optical conjugate relationship through relevant relay lenses 5′.
To perform exposing and framing functions, an optical shutter is set up in the camera bellows. Specifically, in FIG. 1, an aperture diaphragm Qa is set up between the ocular lens 2 and the field lens 3, and the relay lens array 5 is composed of a plurality of relay lenses 5′ which are arranged in an array. Each of the relay lenses 5′ has an exit-pupil diaphragm Qe thereon, thus forming an exit-pupil diaphragm array 8. The aperture diaphragm Qa is designed to form an image Qa′ on a corresponding exit-pupil diaphragms Qe after passing through the field lens 3, the rotating mirror 4, and a transparent glass spherical cover 12. As the image Qa′ of the aperture diaphragm passes upon each exit-pupil diaphragm Qe in sequence by a scan of the rotating mirror 4, each exit-pupil diaphragm Qe is opened accordingly to form an optical shutter for allowing corresponding relay lenses 5′ to finally record the final image I3′ of the mirror image I2′ on the image recording surface 6, thus simultaneously achieving the exposing and framing functions.
People have conducted intensive studies relating to a curved surface where relay lenses and exit-pupil diaphragms are located, and an image recording surface for a long time. These two curved surfaces are generally constructed upon the fungible circle designing theory (SPIE Optical Engineering Press, 1997, Rotating Mirror Streak and Framing Cameras; Acta Photonica Sinica, 2004, Vol. 33, No. 6: 739˜742).
Since the rotating mirror 4 has a certain half thickness r (that is, the normal distance from the rotating mirror axis to the mirror surface), during a uniform rotary scan of the rotating mirror 4, a reflective point T of a secondary image I2 on the rotating mirror 4 continuously changes in such a way that a trace of an image Qa′ of the aperture diaphragm Qa forms a Pascal spiral line. Similarly, the secondary image I2 is mirrored to form a mirror image I2′ which passes through the relay lens to form a final image I3′, a true imaging trace of which is also a Pascal spiral line. As a result, a number of principle errors exist in the rotating-mirror framing camera that is built upon the fungible circle designing theory: (1) Because a curved surface where the relay lens array is located is a fungible-circle curved surface, a defocusing error of imaging points can occur because of inconsistency between this curved surface and the true imaging Pascal spiral line, thus increasing amount of image blur and reducing image resolution. (2) In the above optical shutter, because the trace of the exit-pupil diaphragm array is designed in the form of a fungible circle, a curved surface where the exit-pupil diaphragm array is located cannot be uniformly scanned during a uniform rotation of the rotating mirror, thus causing the framing frequency to become non-uniform. When a large angle is operated, a seriously large time-based identification error may be induced. (3) When designing the curved surface where the relay lenses are located as a fungible circle, a primary ray that passes through a relay lens and its exit-pupil is of a different axis from an optical axis of corresponding reflection, thus leading to an imaging error of the relay lenses and reducing the resolution of the entire camera.
Based on various design needs in existing technology, the above fungible circle design theory can further be divided into defocusing design theory, uniform speed design theory and coaxial design theory respectively targeting the above three deficiencies. However, these design theories can only reduce and minimize a particular principle error. Furthermore, no two design theories can be implemented simultaneously in a same system. Therefore, the above three deficiencies are unavoidable, and a corresponding most preferred design can only be selected according to the specific needs of the system (Acta Photonica Sinica, 2004, Vol. 33, No. 6: 739˜742).